The effect of gamma-aminobutyric acid on brain mitochondrial ATP synthesis

γ-Aminobutyric Acid Is Synthesized and Released by the Endothelium: Potential Implications

RATIONALE

Gamma aminobutyric acid (GABA), a neurotransmitter of the central nervous system, is found in the systemic circulation of humans at a concentration between 0.5 and 3 μmol/L. However, the potential source of circulating GABA and its significance on the vascular system remains unknown. We hypothesized that endothelial cells (ECs) may synthesize and release GABA to modulate some functions in the EC and after its release into the circulation.

OBJECTIVE

To assess whether GABA is synthesized and released by the EC and its potential functions.

METHODS AND RESULTS

Utilizing the human umbilical vein ECs and aortic ECs, we demonstrated for the first time that ECs synthesize and release GABA from [1-(14)C]glutamate. Localization of GABA and the presence of the GABA-synthesizing enzyme, glutamic acid decarboxylase in EC were confirmed by immunostaining and immunoblot analysis, respectively. The presence of GABA was further confirmed by immunohistochemistry in the EC lining the human coronary vessel. EC-derived GABA regulated the key mechanisms of ATP synthesis, fatty acid, and pyruvate oxidation in EC. GABA protected EC by inhibiting the reactive oxygen species generation and prevented monocyte adhesion by attenuating vascular cell adhesion molecule -1 and monocyte chemoattractant protein-1 expressions. GABA had no relaxing effect on rat aortic rings. GABA exhibited a dose-dependent fall in blood pressure. However, the fall in BP was abolished after pretreatment with pentolinium.

CONCLUSIONS

Our findings indicate novel potential functions of endothelium-derived GABA.

Attenuation of evoked field potentials from dentate granule cells by low glucose, pyruvate + malate, and sodium fluoride

Extracellular field potentials were evoked from granule cells of the dentate gyrus by stimulation of the perforant path of the hippocampal slice, superfused with medium containing 10 mM glucose. Decreasing the glucose concentration to 2 mM caused a large but reversible attenuation of the rate of rise of population excitatory postsynaptic potential (EPSP) and of the population spike amplitude. Similar behaviour was observed in the presence of 5 mM glucose, although the decrease in EPSP was much less marked. Substitution of pyruvate + malate (2 mM, 0.2 mM) for glucose in the superfusion medium strongly depressed both EPSP and population spike. Sodium fluoride (1 mM) attenuated the population spike without affecting the EPSP. It is suggested that the behaviour of the evoked granule cell responses in the presence of lowered glucose or alternative substrates does not correlate with the energy state of the tissue.

Brain phospholipid metabolism during ischemia

Molecular mechanisms accounting for the vulnerability of brain to ischemic damage are incompletely understood. To assess the role of membrane integrity, phospholipid metabolism is investigated in subcellular organelles of brain following varying degrees of ischemia. Global brain ischemia is produced in rabbits by combining hypotension and hypoxia (4% oxygen). Restoration of blood pressure and oxygen after a three-minute isoelectric EEG results in full recovery, while a five-minute isoelectric EEG results in either a lack of recovery or neurological deficits. Quantitative analysis of seven phospholipids in subcellular organelles shows no consistent pattern of phospholipid changes following ischemia. Incorporation of radioactive phosphate, on the other hand, discloses moderate alterations following five-minute ischemia. These findings indicate that brain ischemia provokes phospholipid changes, but their role in impairing membrane integrity and in initiating irreversible ischemic brain damage is uncertain.